Can I install Adobe Reader X and still keep Reader 9 also?

Use a virtual machine, like Oracle Virtualbox. have a shared folder.

I have a 32bit virtual machine for using my old scanners and devices that only work on 32bit/16bit windows. You'll have to have virtualization turned on in your bios, and your windows license key will likely work for an install on the inside computer.

You can disable a lot of things on the virtual machine's copy of windows, like uninstall all kinds of apps and services then it loads very quickly and with a shared folder, it is easy to move items back and forth.

Some users find it helpful to mix and match the features and workflows from different versions of Adobe Acrobat to achieve familiar results with their documents. However, as a best practice, only one version of Adobe Acrobat should be installed on a single computer. The presence of two or more versions of the same program on one machine consistently leads to disabled functionality, printer conflicts, and PDF display errors in web browsers, among other problems. Nevertheless, for users willing to accept the risks, installation of multiple Acrobat versions is relatively simple.

Read more about it on: How to Install Multiple Instances of Adobe Acrobat